In general, a nut is a type of fastener with a threaded hole extending through the fastener that is mated with a bolt having an exterior thread to fasten two or more parts together. The combination of the friction between the nut threads and the bolt threads, a slight stretch of the bolt, and compression of the parts holds the parts together. The bolt is under a constant tensile stress called the preload. The preload pulls the nut threads against the bolt threads, and the nut face against a bearing surface of one of the parts, with a constant force, so that the nut cannot rotate without overcoming the friction between these surfaces. In many applications, nut-and-bolt joints are subjected to vibration, which if intense enough, can cause the preload to increase and decrease with each vibration cycle, and may cause the nut to become loose, crack or rupture.
Barrel nuts are specialized nuts, commonly used in aerospace, automobile and other applications that require high torque to bolt parts together, such as hanging aircraft engines from wings and attaching wings or tails to an aircraft fuselage, or in applications where access to the nut is limited. A barrel nut is typically shaped like a round slug or a cylinder. Some barrel nut designs incorporate a partially ‘flattened’ surface to form, for example, a semi-cylindrical cross section. Barrel nuts have a threaded hole extending through the slug in a direction perpendicular to the length of the slug. In use, barrel nuts typically sit inside a hole through a first part and a bolt is threaded into the barrel nut from outside, passing through the second part that is to be bolted to the first part. Barrel nuts are preferred over a standard nut and bolt, when access is limited or not available, because they do not require any modification of the second part or additional materials, thus reducing weight and providing manufacturing and service efficiencies.
An exemplary aircraft field of use for a barrel nut is shown in FIGS. 1A and 1B. An aircraft engine mount 10 is employed to attach an aircraft engine 12 to a strut 14 in an aircraft wing (not shown). The engine mount 10 is a structural element having cylindrical bores 16 extending through the engine mount 10 in a horizontal direction with a central axis H. Bolt holes 20 are positioned through a top surface 22 of the engine mount such that the bolt holes 20 extend into the cylindrical bores 16 with a central axis V perpendicular to the central axis H of the cylindrical bores 16. Barrel nuts 18 are positioned within the cylindrical bores 16 in alignment with the bolt holes 20 to receive bolts 24, which may extend through holes in the strut 14 to attach the strut 14 to the engine mount 10. Once inserted into the cylindrical bores 16, access to the barrel nut 18 is limited. In use, when load is applied, the barrel nut 18 is exposed to stresses from the weight of the engine 12 and movement/flight of the aircraft.